In the prior art of which applicants are aware, U.S. Pat. No. 3,972,392 to Johnson which issued Aug. 3, 1976, discloses clamping the sides of crane rails in a fail-safe mode of operation such that an unpowered or unattended crane is secured to its rails whereby to resist external forces tending to move the crane along the rails. Johnson teaches that a fail-safe clamping apparatus adapted to clamp the sides of crane rails must satisfy several requirements: first, it must be independently powered by fail-safe means such that it is actuated when the crane is unpowered or unattended; second, the clamping assembly must permit sufficient lateral movement of the clamp with respect to the rail to accommodate the movement of the crane wheels on the rail as limited by the tread width between the wheel flanges; third, lateral movement of the clamp assembly should not significantly alter the angle at which the clamping surfaces approach the sides of the crane rails; fourth, where the clamping surfaces comprise metal serrated shoes of hardness substantially greater than that of the rail, means should be provided to prevent the shoes from dragging on the rails when the crane moves along its rails; fifth, the clamping assembly should be of narrow cross section such that the opening in a travelled surface in which the rails are embedded, as in the case of a dock facility, does not exceed approximately three times the width of the top of the rail; and finally, means should be provided whereby remote sensing means are provided to indicate to the crane operator that the clamp is fully released prior to movement of the crane on its rails.
Johnson provides a fail-safe clamping apparatus wherein its clamping surfaces are externally mounted to facilitate periodic examination without disassembling the clamping assembly.
Applicant is also aware of the following co-pending patent applications: PCT application, publication no. WO 2009/092156 and corresponding U.S. patent application Ser. No. 12/018,085, for a Constant Force Rail Clamp. The Constant Force Rail Clamp applications disclose a rail clamp having a frame and a pair of levers. Each of the levers has a brake pad at a first end thereof, a cam follower at second end thereof, and is mounted to the frame by a pivot disposed between said first and second ends. A cam is disposed between the levers and a spring biases the cam in a first direction. A clamp release actuator is operable to displace the cam in a second direction. The second direction is opposite to the first direction. A pair of variably sloped cam surfaces are disposed on opposite sides of the cam. Each of the variably sloped cam surfaces is in contact with the cam follower of a corresponding one of the levers, and each of the variably sloped cam surfaces has a slope which varies to counteract variations in a spring force of the spring as the cam is displaced, thereby maintaining a constant clamping force.